In image sensors of filming equipment such as CCD and displays such as LCD and PDP, a color filter is used to record and reproduce color images. In general, the color filter comprises a transparent substrate, color layers formed on the transparent substrate and made of a color pattern of three primary colors, red, green and blue, and a light-shielding part formed on the transparent substrate so as to divide the color patterns.
Methods such as the pigment dispersing method, the dyeing method, the electrodeposition method and the printing method, are known as the method for forming such color layers. Among these methods, the pigment dispersing method having excellent properties on average, is thus widely used.
In the pigment dispersing method, for example, a photosensitive resin composition for color layers, which comprises a pigment, is provided on a transparent substrate, exposed in a desired pattern and then developed to form color layers in pattern. In the case of forming color layers in this manner, sometimes, the photosensitive resin composition partly remains undissolved and left after development (hereinafter may be referred to as “scumming.”) When the photosensitive color composition remains undissolved and left on non-pixel regions of the substrate, it results in a decrease in transparency or contrast, display defects, etc. Moreover, when the photosensitive resin composition remains on the edge of patterns, there are problems that can affect subsequent processes, such as detachment of ITO layer and deterioration in sealing properties during a liquid crystal cell production process.
In recent years, to improve the color reproducibility of a color filter, color layers are required to have higher transparency and higher concentration. To form color layers with high concentration, it is needed to increase the concentration of the pigment in the resin composition for color layers to be used. However, increasing the pigment concentration involves a problem of difficulty in obtaining pigment dispersion stability. Also, the following problems are caused by increasing the pigment concentration: as a result of a relative decrease in the content of the components which contribute to color layer-forming properties such as photosensitivity and solubility, the layer forming ability that the resin composition inherently has, gets insufficient and the photosensitive resin composition unexposed is likely to remain after development (increase in scumming); and the adhesion between the substrate and a patterned cured layer of the photosensitive resin composition gets poor during development.
To solve the scumming problem in which the unexposed photosensitive resin composition remains, a method has been proposed in Patent Literature 1, in which an organic carboxylic anhydride having a molecular weight of 800 or less, is contained in a color resin composition.
However, even by the method as disclosed in Patent Literature 1, it is impossible to obtain excellent dispersion ability in the case of high pigment concentration and to fully prevent the occurrence of scumming. It is also difficult to obtain a cured product with sufficiently-high adhesion after exposure.
Also in recent years, there is an increasing demand for a display with higher definition, higher luminance and higher contrast, and the use of finer pigments has been studied to meet the demand. However, finer pigments are unstable and pose a problem with dispersion stability. To solve this problem, the use of pigment dispersing agents with high polarity (for example, Patent Literature 2) has been proposed. In this case, however, due to high polarity, applicable solvents and thus the range of applicable members are limited. Commercially-available pigment dispersing agents are also problematic in that the required amount tends to be large, so that problems with development property and adhesion of the layer thus produced are likely to occur and due to the trade-off relationship between pigment dispersion stability and film forming properties, there is a difficulty in increasing the performance of the resin composition.
As the method for uniform pigment dispersion, a method for producing an aqueous solution of a pigment-containing polymer by mixing a polymerization initiator with a monomer-containing aqueous solution for polymerization was proposed (Patent Literature 3). However, when considering the case of applying this method to organic solvent, due to wettability difference between organic solvent and pigment-dispersed water, there is such a problem such that the pigment is likely to settle out. There is another problem such that removal of a pigment-containing polymer from an aqueous solution results in high cost of production.
A method for achieving pigment dispersion stability by adding a specific solvent as a dispersion aid, has been also proposed (Patent Literature 4). However, there is a limit on the dispersion stabilizing effects provided by the use of dispersion aid.
In addition, the pigment dispersion liquid is used in ink-jet inks, toners for electrophotographic printing, writing implements, cosmetics and so on. Even in these fields, dispersion stability of fine pigments is required.